Valve assemblies, such as ball valve assemblies, are well known and have been used for a variety of applications. Such ball valve assemblies commonly include a ball valve that is securely, but rotatably positioned within a housing by a seating or retaining member. Traditionally, the valve assembly and the piping to which the valve assembly is connected have been made of steel or other metal material, but recently thermoplastic piping has become increasingly popular. Thermoplastic piping is desirable for various reasons such as reduced cost, the non-corrosive nature of thermoplastics, and the fact that thermoplastic parts have a longer useful life than do their metal counterparts. Further, thermoplastic piping is easier to install and repair, since adjoining sections of pipe can quickly and easily be butt-welded together. Thus, it is now desirable to also have thermoplastic valve assemblies for use in conjunction with thermoplastic piping.
Designing thermoplastic valve assemblies is more complicated, however, due to the fact that thermoplastic materials tend to expand and contract with variations in both temperature and pressure. The rate and amount of expansion and contraction will depend on various factors, such as the particular type of thermoplastic used, the coefficient of thermal expansion of the thermoplastic, and the physical dimensions of the element. Accordingly, in a thermoplastic valve assembly, the relatively thin thermoplastic housing will typically expand in diameter more rapidly as temperature increases than will other elements of the valve assembly, such as the ball valve or seating member. Thus, during normal operation the physical relationship or the physical "fit" between the various elements of the valve assembly may change. For example, due to their differing shapes and masses, the housing will typically expand radially at a rate greater than that of the ball valve. Under such circumstances, it is difficult to ensure that the fluid seals between the ball valve and the surrounding housing remain intact, since the seating member may expand at a different rate, or if it is physically secured to another element such as the housing, it may move with that element. In addition, in the case of a thermoplastic ball valve, the ball itself, due to its shape, will be particularly subject to thermal or pressure deformation in the areas surrounding its inlet and outlet openings, known as the "lip" regions, since the material thickness is lowest in this area. Deformation of these lip areas will cause flaring that will affect both fluid flow through the valve and the integrity of the ball valve itself. Further, flaring of the lip areas will also prevent the valve from opening and closing properly. Thus, in a thermoplastic ball valve assembly it is also important to maintain the positioning and the structural integrity of the ball valve under all operating conditions.
Known thermoplastic ball valve assemblies, such as that shown in FIG. 1, have provided a thermoplastic housing 100, with thermoplastic seating members 101 that are positioned within a recess or groove 102 in the housing itself, and thus are physically secured to the housing. Such a configuration does not solve the problems described above, since the movement of the housing (such as by expansion) directly corresponds to movement of the seating members. Thus, as the housing expands, the seating member that is secured to the housing tends to move with it, being drawn away from the ball valve. As described above, this has the disadvantage of affecting the fluid seals between these elements, and also decreases the ability of the seating member to ensure the proper positioning and the physical integrity of the ball valve, particularly around the lip areas, under all operating conditions.
Thus, known thermoplastic ball valve assemblies have been unable to account for large variations in temperature or pressure, and therefore, have been limited to applications involving relatively mild operating conditions, i.e., uses under which temperature and/or pressure variations are relatively insubstantial. These devices are simply unsuitable for many applications in which thermoplastic piping is otherwise desirable, such as the water market, where operating conditions as well as testing requirements are much more severe.